Blender Git Loki

IMB_metadata improvements

- Metadata handling is now separate from `ImBuf *`, allowing it to be
used with a generic `IDProperty *`.
- Merged `IMB_metadata_add_field()` and `IMB_metadata_change_field()`
into a more robust `IMB_metadata_set_field()`. This new function
doesn't return any status (it now always succeeds, and the previously
existing return value was never checked anyway).
- Removed `IMB_metadata_del_field()` as it was never actually used
anywhere.
- Use `IMB_metadata_ensure()` instead of having
`IMB_metadata_set_field()` create the containing `IDProperty` for
you.
- Deduplicated function declarations, moved `intern/IMB_metadata.h` out
of `intern/`. Note that this does mean that we have some extra
`#include "IMB_metadata.h"` lines now, as the metadata functions are
no longer declared in `IMB_imbuf.h`.
- Deduplicated function declarations, all metadata-related declarations
are now in imbuf/IMB_metadata.h.

Part of: https://developer.blender.org/D2273

Reviewed by: @campbellbarton